Estimate would be better then no price yes, just so long as you can figure out a formula where your estimate ends up within no more then say $10-20 of the final price I think most would be happy enough with that.
Either that or you just set a price and if the item is cheaper good for eveyone but if it's more you would probably in fairness end up paying the difference out of pocket, which is unfortunately the price one has to pay if they are selling an item that doesnt have a final price (and why this type of business can be tricky to balance).
Remember to shop around with post carriers as you have more options then AusPost too.
Also if you use http://www.transdirect.com.au/ you can get quotes forr multiple couriers at once.
And lastly for parcel dimensions when in doubt look at the dimensions of similar sized items, if you compile a list of approximate sizes and weights you should be able to also figure out a postage price list to add to your first post so that people have a rough idea what each thing will cost to send